With stretch-bend leveling in a stretch-bend leveler of this type or a stretch-bend leveling stand of this type, the strip as a rule is under tension below the elastic limit or yield point and the strip is bent around the straightening rollers in the plastic or elastic-plastic range bidirectionally. The straightening rollers acting in a plastic or elastic-plastic manner are also referred to as stretch rollers. The amount by which the strip (overall) is plastically stretched and is consequently elongated is referred to as the stretch ratio.
With a stretch-bend leveler of this type nonplanar metal strips can be straightened and consequently nonplanarity can be eliminated. Nonplanarity means, for example, strip waviness and/or strip camber resulting from length differences of the strip fibers in the strip plane. However, nonplanarity also means strip curvature in the longitudinal and/or transverse direction resulting from bending moments in the strip, for example, if the strip was elastically/plastically bent around deflection rolls, or is caused by elastic-plastic deformations when winding up the strip. Longitudinal curvatures are also referred to as coil set, transverse curvatures as cross bow. In the course of stretch-bend leveling, the nonplanar strip is bent (bidirectionally) around rollers with a sufficiently small diameter under a tension that is below the elastic limit RE or the technical elastic limit Rp0.01 of the strip material, so that with the superimposition of the tensile stress with the bending, an elastic/plastic deformation of the strip is generated. The strip is plastically elongated, the amount of the plastic elongation being referred to as the stretch ratio. In the case of the plastic elongation, the originally short strip fibers are elongated to a relatively greater extent. In the ideal case after straightening all of the strip fibers have the same length so that in principle an ideally straightened strip free from waviness or strip cambers should be produced. This is not always achieved fully in practice, so that slight residual nonplanarity (for example, center waves or edge waves) can remain in the strip. Moreover, due to the bidirectional bending, residual bending moments are induced in the strip that can lead to undesirable plastic residual curvatures after straightening in the longitudinal direction (coil set) or transverse direction (cross bow). Through suitable coordination of the bending intensities on the individual rolls, the residual bending moments can be basically minimized. To this end it has already been proposed in different ways to adjust the geometry of the stretch-bend leveling stand to achieve the best possible straightening results.
Moreover, it is basically known from practice to provide deflection rolls upstream, between and/or downstream of the straightening rollers, which deflection rolls have a much larger diameter than the straightening rollers and generally have only an elastic effect.
Thus a device for leveling a metal strip is known, for example, from EP 0 298 852 (or DE 38 85 019) [U.S. Pat. No. 4,898,013], in which several stretch-bend leveling rollers are provided between a set of input tension rollers and a set of output tension rollers. Deflection rollers can be provided at different locations of the apparatus, which deflection rollers are primarily used to guide the strip during its travel through the apparatus along a certain path.
In a similar manner the professional article “Benefits of a new leveler technology for packaging steels: Multi-roller tension leveler” (Emmanuel Dechassey, Irsid, Arcelor Group, METEC Congress June 2003) describes a stretch-bending-leveler with four straightening rollers. There the overlap of the first straightening unit (first and second straightening roller) and the overlap of the second straightening unit (third and fourth roller) are movable. In turn several deflection rollers with larger diameter are integrated into the apparatus.
The stretch-bend levelers known from practice, in which deflection rollers with elastic action are also provided optionally, have proven to be basically useful, but can be developed further.